Field of the Invention
The invention relates to an actuator having at least one control element which has thermally activatable transducer material. The at least one control element changes from a first shape state into a second shape state. A mechanical energy storage is functionally connected to the at least one control element and when the at least one control element is in the second shape state it exerts a restoring force on the at least one control element which returns the at least one control element to the first shape state.
Description of the Prior Art
Actuators having at least one control element which utilize thermally activatable transducer material are used to perform mechanical actuation in which the control element is to be deflected in a controlled, bidirectional manner along a movement trajectory, which in most cases corresponds to along a linear axis. For a multiplicity of technical, in particular control applications, control elements are used which utilize a shape memory alloy, often in the form of shape memory wires or plates.
Such control elements are generally activated either by the ambient temperature or by artificial thermoelectrical stimulation. In the case of artificial thermoelectrical stimulation, an electrical voltage is applied to the control element including an electrically conductive shape memory alloy, (SMA). The voltage results in a flow of electrical current along the control element producing a resistance-induced ohmic heat, which heats the control element.
As a result of the heating, SMA control elements generally undergo a change in shape, preferably in the form of a shortening or change in size, which is caused by a phase transformation between the martensitic low-temperature phase to the austenitic high-temperature phase. Many of the known shape memory alloys cannot perform any more work once the austenitic high-temperature phase has been achieved. In these cases an automatic reverse deformation once the shape has changed when the shape memory alloy is heated does not occur, even when the temperature is reduced.
In order to be able to perform work again in these cases, the control element must be returned to the starting state, in which an SMA must be cooled and also deformed back to the starting state by means of an external mechanical force. It should be noted that such a restoration of shape induced by a mechanical external force is also used for shape memory actuators which can be activated correspondingly by magnetic fields.
The mechanical restoration of SMA control elements which have been deflected once or deformed once by thermal activation, for example in the form of wires or wire bundles, takes place with the aid of restoring elements functionally connected to the control elements, for example in the form of springs or weights. During the switching process, the control element thus performs mechanical work induced by a change in travel, which is partially used to deform the corresponding restoring element, for example to tension a spring or to displace a weight, for example to lift a weight counter to gravity. During restoration, the mechanical energy stored in this manner is transmitted back to the control element, as a result of which the latter can be transferred to its starting state.
A further possibility for restoring SMA control elements to the starting state can also be achieved with a plurality of SMA control elements entering into alternating functional connections, in which the switching process of an SMA control element causes a corresponding restoration of another SMA control element. Restoring elements in the form of springs and weights can be omitted in this manner.
The prerequisite for complete restoration, which can also be achieved within a short time, of an SMA control element deformed by thermal activation depends critically on the ability to conduct away or dissipate the thermal energy necessary for the single deformation process in the form of heat out of the control element after the switching process.
To support the cooling process, the convection-induced heat removal process is supported in a known per se manner, for example with the aid of an external fan, as a result of which the convective cooling process is greatly enhanced and the cooling speed of the SMA control element can be increased, in order in this manner to allow a faster return to the starting state and thus a faster repeated switching or activation of the control element.